1. Field
The invention is related to circuits for separating chrominance and luminance signals in color television processing and systems, and particularly to a process and apparatus for precluding visibly objectionable flicker at vertical transitions in a television picture signal, when reproducing both frames of a composite color video signal from only a single frame or field.
2. Prior Art
Video storage systems with self-refreshing outputs are available which selectively store color television signals for rapid retrieval. Typical of such systems is the HS-100 color slow-motion recorder/reproducer, manufactured by Ampex Corporation, Redwood City, California. In such system, an NTSC color television signal is generated from a single magnetically stored PCM video field. Various other systems provide the reproduction of a composite color video signal from a single stored frame, wherein "frame" is hereinafter defined as two interlaced fields of a four field color coded NTSC color television signal. Because of the characteristics of the NTSC color television signal, in order to generate a composite television signal from a single stored field or frame, it is necessary to separate the luminance and chrominance signals, invert the chrominance on alternate frames, and then recombine the two signals. The chrominance separation process is generally accomplished utilizing a comb filter and bandpass filter, previously in the analog format, but more recently in the digital format. As is well known, the comb filter takes advantage of the frequency relationships between the horizontal line rate and the color subcarrier signal, and typically utilizes three adjacent television lines in a given field and selectively adds and subtracts them to obtain the separated chrominance and luminance signals.
The disadvantage of the separation process is that at places in the television picture where there are vertical discontinuities, e.g., at color edges, the comb filter inherently does not completely separate the chrominance and luminance signals. At these color edges, a portion of the chrominance is extracted to the chrominance channel, while the remainder of the chrominance is left in the luminance channel. When reproducing the video, chrominance inversion is performed on alternate frames by changing the sign of the signal in the chrominance channel. Subsequent addition of the chrominance to the signal in the luminance channel provides incomplete recovery of the chrominance in the vicinity of the vertical discontinuities. The result is one frame with full chrominance saturation and sharp color edges, followed by an alternate frame where vertical discontinuities have de-saturated edges. This causes a readily visible and objectionable 15 Hz flicker in the picture at the color edges.
As previously mentioned, the process of reproducing a composite color television signal from a single field or frame of stored video employs some form of comb filtering, a bandpass filter for removing low frequency chrominance components, and means for performing an inversion of the chrominance component on alternate frames. Various digital comb filters and bandpass filters have recently been made available for providing the chrominance and luminance signal separation. However, the process and apparatus for re-combining the chrominance and luminance through some form of digital inversion, is not presently available. A fully digital system is preferable to provide the desired signal stability and reliability conventionally associated with digital implementation.
In reproducing the composite color television signal from a single field, as in a slow and stop motion recorder/reproducer, the separation, the bandpass filtering and the inversion processes are presently performed via analog implementation. In such systems, it would be very desirable to provide the above processes in a digital format, with digital implementation.